1. Field of the Invention
This invention relates to an electronic flash unit, and more particularly to an electronic flash unit in which the noise produced during the excitation of the gas in the flash discharge tube is reduced.
2. Description of the Prior Art
Various electronic flash units have heretofore been proposed. For example, there is an automatic flash output control type electronic flash unit provided with a flash discharge tube having xenon gas or the like enclosed therein, a main capacitor charged with electrical energy for flash discharging, a trigger circuit for exciting the flash discharge tube at a high voltage in synchronism with the operation of a synchro contact (X contact) and starting flashlight emission by the charged energy of the main capacitor, and a light emission stopping circuit for stopping the flashlight emission when the quantity of emitted flashlight reaches a predetermined value (U.S. Pat. No. 4,228,381). Every electronic flash unit, including one of the automatic flash output control type, has a trigger circuit for causing the flash discharge tube to emit light, and vicious noise is discharged during operation of this trigger circuit. On the other hand, the latest cameras are advanced in automatization such as automatic exposure control, automatic focus adjustment or the like and along therewith, the electronic circuits are complicated and such cameras are sensitive to noise. Likewise, electronic flash units are also advanced in electrification and are sensitive to noise. Therefore, from the standpoint of preventing malfunctioning, improved noise margin of each circuit and a shield for intercepting the noise become necessary to reduce the influence of the noise discharged from the trigger circuit. This leads to a problem that the electronic flash unit and the electric circuits of cameras become expensive.
Also, the flashlight emission of the flash discharge tube is affected by the brightness of the outside. That is, if the flash discharge tube is placed in a light environment, the gas enclosed in the flash discharge tube is excited by light and it becomes easier for the flash discharge tube to emit light and conversely, if the flash discharge tube is placed in a dark environment, the excitation by light becomes null and it becomes relatively difficult for the flash discharge tube to emit light.